1. Field of the Invention
The present invention relates to a system and device for dynamically adapting a read channel equalizer.
2. Description of the Related Art
Magnetic tape cartridges include magnetic tape to store data to be saved and read back at a subsequent time. A magnetic tape drive writes the data to magnetic tape, typically as a set of parallel tracks, and subsequently a magnetic tape drive reads back the data. To read back the data, a magnetic tape drive typically comprises parallel read heads to read each of the parallel tracks, a drive system for moving a magnetic tape with respect to the read heads such that the read heads may detect magnetic signals on the magnetic tape, and a read channel for digitally sampling magnetic signals detected by the read heads and providing digital samples of the magnetic signals. The digital samples are then decoded into data bits, and the data bits from the parallel tracks are combined into the data that was saved. The read channel typically requires an equalizer for each of the read heads to compensate for the change in the signal due to the magnetic recording properties of the write head, the magnetic tape, and the read head. Magnetic tapes may be interchanged between tape drives, such that a magnetic tape written on one tape drive will be read by another tape drive. Variation in the response of the read heads to the variously written magnetic tapes may result in unacceptably poor read back of the recorded signals.
Adaptive equalizers implemented in magnetic tape drives solve a set of equations to find the equalizer characteristic that reduces the error between the desired and actual amplitudes. The set of equations may be highly complex and computationally expensive. Thus, the equalizer might be computed at the beginning of use with respect to a magnetic tape, or recomputed a few times during use. Further, the desired amplitudes may be difficult to estimate. Hence, in many instances, the desired amplitudes are best estimated by employing a signal having known characteristics, such as a synchronization signal, or a data set separator signal, and not the random data signals.
In magnetic tape, the recording characteristics may not only vary from track to track, but may as well vary in a continuous fashion along a track or tracks. Thus, a selected equalizer characteristic, although satisfactory at the beginning or at some specific track location of a magnetic tape, may lead to an increase in data read errors at some point along the track.
Further, in magnetic tape, an equalizer typically equalizes signals in the asynchronous domain, which means that the digital samples that are processed by the equalizer are taken asynchronously with respect to the clock that is used to write the data on the magnetic tape. This makes a determination of a desired amplitude at the point of the asynchronous sample a difficult task.
The co-pending and commonly assigned patent application entitled “Dynamically Adapting a Magnetic Tape Read Channel Equalizer”, by Evangelos S. Eleftheriou, Robert A. Hutchins, Glen Jaquette, and Sedat Oelcer, having application Ser. No. 11/003,283 and filed on Jan. 12, 2005, provides a technique for dynamically adapting the equalizer to improve stability and the signal-to-noise ratio. In this application, the equalizer has at least one adjustable tap and equalizes input read signals. A detector senses the equalizer output signals after the gain has been adjusted by a gain control loop. The received signal and the desired signal are typically different and the difference is used to produce an error signal that is provided as feedback to a gain control loop that adjusts the variable gain amplifier circuit that controls the amplitude of the equalizer output signal. The same error signal is further provided to an equalizer adaptor to feed back sensed amplitude independent errors to adjustable taps of the equalizer. Further, if the gain control loop and the equalizer adaptation loop use error signals from the same source, the two loops interact. The result of this interaction is that more taps in the equalizer must be fixed for stable equalizer loop adaptation. However, fixing more taps reduces the ability of the equalizer to adapt.